It has become common practice to establish a conference connection of n telephone lines using space division technology. This technology uses metallic contacts, contained in an array of relays or cross-bar type switches, to establish a telephone connection between a first subscriber and a second subscriber. Often, as additional subscribers were added to the space division connection, the voice quality of the connection was degraded due to the loading of the individual lines on the connection.
This problem was alleviated somewhat by limiting the number of connected conferees and by using operational amplifiers with resistor networks to build low impedance summing bridges.
Currently, it is common for telephone connections to pass through some form of digital time shared circuitry or channel, such as the Bell System's well known "T1" carrier system, a time division multiplexing switching system.
Time division multiplexing is a type of digital switching in which a line is time shared among multiple users, and in which multiplexed time slots are allocated for establishing communication sessions.
The art of digital time division switching also includes systems known as Time Slot Interchangers (TSI), in which digital message samples from a given input time slot are switched to a different output time slot. TSI is now used to establish two party connections.
U.S. Pat. No. 4,119,807; entitled "Digital Time Division Multiplex Switching System" and issued Oct. 10, 1978; is an example of a time shared digital switch arranged to establish conference connections without restriction as to the number of conferees that may participate in a conference session. This patent discloses an arrangement for sequentially summing digital message samples from 128 lines connected respective to 128 time slots during a first time frame and outputting the summed samples to each line of the connection, respective to its assigned time slot, during a second time frame.
A digital switching arrangement of this type uses two summation memories, which are alternately loaded and unloaded respective to a time frame. In addition, third and fourth memories are alternately loaded in step with the summation memories, and are used for the storage of digital message samples respective to assigned time slots. Each summation memory is cleared at the beginning of its respective storage cycle or time frame in preparation for the storage of a new series of summed message samples.
Digital switching systems disclosed by the prior art, including U.S. Pat. No. 4,119,807, are arranged more or less to process 128 time slots at the input and 128 time slots at the output. These arrangements are not readily integratable onto a single integrated circuit chip, especially if the number of time slots are increased to 256. The problems of integrating a circuit of this size onto a single chip occur as a result of increased memory requirements, and corresponding increase in the number of circuit elements. The problem is particularly highlighted if additional functions, such as conference broadcast and monitor arrangements, are added to the system.
Further, a digital switching system must account for signal propagation delay due to cascaded circuit elements before the size of a digital switch system is arbitrarily doubled. Such signal propagation delay could result in a loss of incoming digital signals if not properly addressed. The memory requirements for a digital conferencer could be proportionately reduced by reducing the number of bits used to digitally encode voice signals. However, such a reduction in the number of bits used to encode audio signals will proportionately degrade the final resolution of digitally encoded voice signals back into analog voice signals. In fact, a well designed digital conferencing circuit uses more than eight bits to digitally encode audio signals for improved resolution of decoding digitally encoded signals back to analog signals.